The term "flat-panel display" as used herein refers to field emission displays (FEDs) and other flat-panel displays, such as addressed in Tannans, Flat-Panel Displays and CRTs (1985 Van Nostrand Reinhold). In this context, the term "flat" has reference to thinness, not planarity.
Flat-panel displays are widely used as imaging screens for laptop and notebook computers, but are not limited to such applications.
Conventional image display devices employing luminescent materials suffer from inefficiency in image generation. The directivity of light emitted by phosphor particles, for example, is generally random and uncontrolled. A portion of the total generated light is lost due to back emissions that never reach the viewer. The phosphor of luminescent displays is conventionally deposited over a smooth, slightly curved or planar surface, oriented generally normal to the incoming electron stream. Thus, to avoid loss of image due to passage of electrons between adjacent phosphor particles, the phosphor is deposited in multiple layers, so outer layer particles cover interstices between inner lay particles. This increases the operating power requirement in FED displays, however, because of higher resistance paths between outer layer particles and the associated anode stripes. Moreover, also relating to FED displays, it is well-known that phosphor light emission efficiency decreases with incident electron current density. This problem exists with traditional CRT displays but is especially troublesome with flat-panel displays which typically operate at lower power and under often brighter ambient light conditions. It is, therefore, desirable to be able to construct an anode plate for an FED image display which has increased phosphor surface area, without sacrificing high resolution pixel size.
Flat-panel displays also suffer from contrast ratio reduction and glare due to reflections of ambient light from the face plate. This is of particular concern with displays employing face plates having phosphor luminescent coatings because such displays are subject to much greater contrast ratio reduction due to reflections of ambient light from the anode stripes and granular phosphor. It is, therefore, desirable to be able to construct an anode plate for an FED image display which has reduced ambient light reflection, without sacrificing image intensity.
Various structures and treatments have been proposed to address the problem of ambient light reflection, including the provision of surface irregularities and patterns which function as ambient light scattering elements to redirect reflections of the incident ambient light out of the angle of view of the viewer. An example of such treatment is given in U.S. Pat. No. 5,240,748 wherein a shallow pattern of 0.1 .mu.m depth is ablated by UV light on the inside surface of a CRT display. However, though scattering reduces reflections at certain viewing angles, non-productive light (i.e., light that is not part of the image-formative process) is still returned to the viewer.
U.S. Pat. No. 5,206,746 discloses a transparent plate having a rear surface with a side-by-side array of triangular prisms that is interposed as a unidirectional light trap between liquid crystal and backlighting components of a liquid crystal display. Ambient light incident on the bottoms of the prisms is internally reflected at the prism side surfaces and directed toward the tops of the prisms where it is absorbed by a coating of light absorbing material. Light traveling in the opposite direction from the backlighting source is, however, relatively unaffected and passes through to the viewer, or is blocked, in accordance with the pass/no-pass mode imparted to the liquid crystals. The '746 structure constitutes an independent element, separate and apart from the active image-forming liquid crystal and backlighting components.
Applicant's copending application U.S. Ser. No. 08/347,011, entitled "Ambient Light Absorbing Face Plate For Flat Panel Display," discloses a transparent face plate for a cathodoluminescent display having a rear surface prism array, wherein tops of the prisms are covered not only with light absorbing material, but also with electrically conductive material. The conductive material is connected to serve as anode stripes for excitation of phosphor granules deposited over the coated tops. This arrangement enables the compact construction of an FED display having improved contrast ratio and reduced electrical surface leakage between adjacent different colored phosphor stripes.
The full disclosures of U.S. Pat. Nos. 5,206,746 and 5,240,748, and of copending U.S. Ser. No. 08/347,011, are incorporated herein by reference.